Location of conducting and/or magnetic bodies



July 17, 1951 s. wHrrEHEAD Erm. 2,550,834

LOCATION OF CONDUCTING AND/OR MAGNETIC BODIES Filed Jan. 9, 1946 4Sheets-Sheet 1 STANL; y' WHI'TSHEAD BLNJ'AH\N RasEnBLuM lNvlN-rols s:wHl'rEHl-:AD ErAL LOCATION OF OONOUO'IING ANO/OR MAGNETIC BODIES FiledJan. 9,1946

Jlu1y11, 1951 l4 sheets-sheet 2 July 17, 1951 s. WHITEHEAD ErAL2,560,834

LOCATION oF coNDucTING AND/0R MAGNETIC BODIES Filed Jan. 9, 1946 4Sheets-Sheet 5 53 @j @f6 (a) Q @y0 o 0 NIAMm RostwbLuH NvENToes July 17,1951 S. WHITEHEAD EI'AL LOSATION OF CONDUCTING AND/0R MAGNETIC BODIESFiled Jan. 9, 194e 4 Sheets-Sheet 4 STANLev wmenemo Einsam N RoseusLvn\NVLNT|`S (i1-1 @www4 Patented July 17, 1951 LOCATION F CONDUCTING AND/OR MAGNETIC BODIES Stanley Whitehead and Benjamin Rosenblum, London,England; said Rosenblum now vby change of name Benjamin RustonApplication January 9, 1946, Serial No. 639,986 In Great Britain January24, 1945 The present invention relates to the location of conductingand/or magnetic bodies, and particularly by the use of electro-magneticmeans.

It is often desired to find objects which are hidden in a greater orsmaller volume in space. often when they are buried or otherwisesituated under the surface of the earth. In such a search advantage isgenerally taken of some property by 'which the objects diier from theirsurroundings. When` dealing with conducting and/or magnetic bodies, suchas ore bodies, pipes, cables, joint or junction boxes, tanks, cisternsand the like, the electro-magnetic method of location may be employed.The principle of this method is to produce an alternating magnetic eldover the volume to be searched and to detect the distortion in the fielddue to the presence of the body to be located by reason of its magneticpermeability and/or the eddy currents induced in it. Y

The iield may be produced only in the immediate vicinity of search orelse it may be generated over a larger volume.

In the former application a two-coil locator is generally used, Whereone coil sets up a local magnetic field while the other serves as thesearch coil. This type can only be used in locating objects at a smalldepth below the surface of the ground, since the locator sensitivity isinversely proportional to the sixth power of the distance from theobject. In addition interference between adjacent operators may occurunless they are provided with equipment of separate and individualfrequencies.

Where sensitivity to objects placed at a greater depth is desired, themagnetic fleld is generated over a larger volume. One easy way ofproducing a magnetic iield over a large volume is by means of analternating current passing through a conductor. The return path of thecurrent may either be through the soil or through another conductor. In.the latter case a loop is formed which may consist of several turns, andit may be placed either outside the area of search or it may surround apart or the whole of the area.

The problem of location of metallic and/or magnetic objects would undersuch conditions be simplified if a uniform field could be produced, butthis is generally impracticable. For this reason the method hithertoemployed in prospecting has been to measure the magnetic eld at a numberof points by means of one or two search coils. The results are plotted,and from irregularities in the field the presence of searched objects isdeduced.

' It is the object of .the present invention to pro-` 'z claims. (ci.11s-182) vide search equipment to make such searches more direct and toenable them'to be carried out expeditiously in spite of the use of anon-uniform magnetic eld.

According to the present invention there is provided equipment for thelocation oi electrically conducting bodies and magnetic bodies,comprising means, including a fixed electrically conducting loop andalternating current generating means, for producing an alternatingmagnetic eld over a relatively large area, and search equipment forsearching over said area and having dimensions which are smallrelatively to said loop, the search equipment comprising two groups ofsearch coils each having a plurality of turns, each said group having atleast one coil and the sum of the areas enclosed by the individual turnsof the coils of one of said groups projected on to a iirst referenceplane containing one of the turns being substantially equal to the samesum for the coils of the other of said groups, thesame reference planebeing used for both said groups, a

larea-turns moments of coils of one of said groups substantially equalto the algebraic sum of the area-turns moments of the coils of the otherof said groups, said area-turns moment for each of said coils being theproduct of (a) the sum of the areas enclosed by the turns of the coilprojected on to said iirst reference plane and (b) the distance of thecentre of the coil from any axis in said second reference plane, saidiirst reference plane and said axis being the same for all said coils,and a voltage responsive device connected to indicate the differencebetween the electromotive forcesgenerated by said eld in the coils ofsaid two groups.

Other objects and features of the invention will be apparent from thefollowing description in conjunction with the accompanying diagrammaticdrawings in which Fig. 1 shows by way of example one arrangement forproducing an alternating magnetic i'leld in an area to be searched,

Fig. 2 illustrates one embodiment of the invention by way of example,

Fig. 3 is a view of a part of a search unit in sectional front elevationindicating the manner in which the search coils in Fig. 2 can bemounted,

Fig. 4 is a circuit diagram of one form of amplilier and compensator foruse in Fig. 2,

Fig. 5 shows certain alternative arrangements of search coils,

Fig. 6 illustrates a partof a further embodiment of the invention,

Fig. 7 shows more clearly the circuit arrangement of Fig. 6,

Fig. 8 is a view in sectional side elevation of a part of another searchunit indicating the way in which the search coils in Figs. 6 and 7 maybe mounted,

Figs. 9 and 10 are diagrammatic sketches illustrating alternativearrangements of search coils shown in Figs. 2 and 3.

Like parts are given the same references in the several iigures.

Referring to Fig. 1 an area. I0 to be searched lies outside a loop ofcable II. A single turn loop is shown but two or more turns may be usedif desired. By way of example the loop may measure 150 x 300 yards. Agenerator I2 is connected to feed to the loop an alternating current ofa suitable frequency, for example 500 cycles per second. The generatorI2 may be of 6 kilowatts. The current in the loop generates in the areaIll an alternating magnetic ileld which is distorted in theneighbourhood of conducting or magnetic bodies.

is that within the loop.

. Referring to Figs. 2 and 3, a search coil unit comprises four searchcoils I3, I4, I and I6 wound in the same sense, mounted in the sameplane upon a support upon which they can be rigidly fixed with theiraxes parallel and coplanar. Two compensating coils I8 and I9 are mountedupon the support 20 with their axes at right angles to those of thesearch coils and mutually perpendicular. The outer terminals of all thecoils I3 to I6 are connected together and to an adjustable tap 23 on animpedance 24, in this case a resistor having two outer terminals 25 and26. The inner terminals of the two outer lcoils I3 and I6 are connectedtogether and to the terminal 25 whilst the inner terminals of the twoinner coils I4 and I5 are connected together and to the terminal 26. Acapacitor 38 is connected between the point 23 and an adjustable tappingpoint 39 yon a resistor 31 which is connected in parallel with theresistor 24. The arrangement, constitution and connection of the coilsI3 to I6 are such that the coils form a balanced system so that whenthese coils are traversed by like magnetic fields, or when they aresituated in a uniform alternating field, two potentials tending tobalance each other are applied between terminals 25 and 23 and between26 and 23 respectively. Thus the potential difference between terminals25 and 26 is substantially zero or at least considerably smaller thanthat obtained between 25 and 23 or that obtained between 26 and 23. Byadjustment of the tappings 23 and 39 on resistors 24 and 31respectively, and, if necessary, by slightly displacing one of the coilsI3 to I6 relatively to other coils, it is arranged that there issubstantially zero input to the amplifier due to the alternatingmagneticfield and the rst space differential of the ileld as determinedby moving the search unit in horizontal and vertical planes landrotating it about a vertical axis in the magnetic eld area of search.The magnetic Iield space differential of nth order at any point in spaceis factorial n times the increment in magnetic field at that point abovea datum level, the increment being due to a spatial iield variation ofthe nth order Thus is the magnetic iield at a point distant a from afeeder,

It will be understood that it may if desired be arranged that the areato be searched 4 then the eld at a point (a4-6a) from the feeder is:

. lnH nl where WH is the magnetic field space dlierential of nth order,and may be expressed by:

Each coil I3 to I6 may have connected in series with it and disposedco-axially within it a, small trimming coil 21, 28, 29 and 30respectively for the purpose of assisting the matching the coils exactlyand so enabling the balance above referred to to be obtained.

In one example the coils I3 to I6 are each of 71/2 inside diameter, 8%"outside diameter and 0.2" in axial width; they are wound with 660 turnsof 32 S. W. G. single silk covered enamelled copper wire and have aself-inductance of milllhenries. Their centres are 1 foot apart.

A part, approximately one half, of a Search unit embodying search coilssuch as are shown in Fig. 2 is shown by way of example in Fig. 3. Ahousing of insulating, non-magnetic material has an upper part 20 and acover plate 2| bolted thereto. Search coils I3 and I4 only are shown andeach is xed. for example by adhesive, to an insulating plate 22. Theplate 22 of coil I3 has elongated apertures 12A through which pass bolts12 whereby the coil I3 can be firmly clamped to the housing part 20 butcan be slightly adjusted relatively to the other coils. The co'il I4 hasits plate 22 xed to the housing 20 by adhesive. The other coils may befixed similarly to coil I4 or one or both may beadjustable as coil I3.The coils I8 and I9 are mounted by brackets 13 and 14, respectively,above the part 22 and are provided with a cover 15. Two hinged handlesare provided, a part of one being shown at 16.

When the oo il system I3 to I6 is immersed in an alternating magneticeld which is distorted by the presence of a conducting and/or magneticbody, the fields traversing the coils are not the same and a differenceof potential is produced between terminals 25 and 26. For the purpose ofindicating this difference of potential or out-ofbalance, there areprovided as shown in Fig. 2 an amplifier 3| and an indicating device 32,such as a telephone receiver for example. The points 25 and 26 areconnected to the amplifier input through resistors 52 and 53respectively.

In the circuit of Fig. 2 so far described, variations in the meanmagnetic field traversing the search coils will aiect the voltageapplied to terminals 25 and 26 and hence the indication given at 32. Inorder to reduce or eliminate this effect there may be provided acompensator 33. The input voltage for the compensator is derived from abalancing circuit comprising the resistors 24 and 31 and capacitors 38,34 and 35. The joint 40 of the capacitors 34 and 35 is earthed, and thevalues of the capacitors 34 and 35 are arranged to be substantially lessthan those necessary for producing resonance with the coils I3, I4, I5and I-6 at the frequency of the alternating magnetic field.

The potential difference between points 23 and 40 is approximatelyproportional to the mean magnetic eld in the region of the coil unit andthis potential difference is applied to the input of the compensator 33.A circuit for the amplier 3I and compensator 33 is shown in Fig. 4within two dotted rectangles. The amplifier comprises three variable-muamplifying valves ViyVn, and V3 and has input terminals 4| and 42. Thecompensator comprises a variable-mu amplifying valveV4 and has inputterminals 43 and 44. The output of the valve V4V is rectified by arectiiier 45. A part of the rectified output is tapped off fromresistors 50 and 5|, smoothed in a circuit 4B and applied through a lead41 as a bias to suitable grids of one or more of the valves of theamplifier 3|, in this example to the control grids of the Valves V2 andVs. The D. C.

Voltage developed across the rectifier 45 'is also fed back to thecontrol grid of the valve V4 through a smoothing circuit comprising aresistor d8 and condenser 49.

The valve V4 is preferably chosen to have the same characteristics asthe valves V2 and Vs of which it controls the amplication so that therelation between the control bias applied through lead 41 and theamplication of the valves V2 and Va is substantially identical with thesame relation in the compensator valve V4 By suitably adjusting theratio of the D. C. voltage fed back to the grid of the valve V4 to theD. C. voltage applied to the grids of the valves V2 and Va,

. the amplifier output can be made independent of the mean fieldstrength. Although one form of compensator has been described by way ofexample, other known or suitable compensating means may be provided.

With the arrangement of four search coils I3 to -I6 described and shownin Figs. 2 and 3, there is substantially zero input to the amplifier 3ldue to the -alternating eld generated in the area lil of Fig. 1 when thesearch coil unit is in this iield.

There is also substantially zero input to the arnpliier due to the rstspace differential of the eld determined by moving the search unit inhorizontal and vertical planes and by limited rotation thereof about a,vertical axis parallel to the coil axes.

Substantial balance so far as forward or lateral rotation of the searchunit about horizontal axes can be secured with the aid of the coils I8and I9. The coil I8 is connected through a reversing switch 54 acrossthe resistor 52 and the coil I9 isy connected through a reversing switch55 across the resistor 53. Voltages from the coils IB 4and I9,adjustable by means of the tappings on the resistors 52 and 53 aretherefore applied between terminals 25 and 4I and terminals 26 and 42respectively. Any phase difference between these voltages and thatdeveloped between terminals 25 and 26 can be reduced by changing thevalue of the resistor 36 in Fig. 2. 'In order to adjust the unit, thetappings on the resistors 52 and 53 Iand the reversing switches 54 and55 are adjusted until the desired balance is obtained when the unit isgiven limited rotary movements about horizontal axes.

In the arrangement described the electromotive force applied betweenterminals 4l and 42 will still give an indication at 32 when the coilunit is rotated in such a way as to generate an electromotive forcedependent on the second and higher space differentials of the magneticiield but the magnitudes of these drop rapidly with increase in distancefrom the cable Il in Fig. 1. If desired space differentials of secondand higher orders may be substantially balanced by the use of a multipleof four coils.

Thus referring to Fig. 5, at a is shown the coil arrangement of Fig. 2,the mode of connection of the coils vto the resistor 24 being indicatedby a circle and a cross within a circle. fI'hus coils I3 and I8 whichhave their inner terminals connected to terminal 25 and form one groupof coils are indicated in Fig. 5 by a plain circle and coils I4 and I5which have their inner terminals connected to terminal 26 and form asecond group are indicated by a cross within a circle.

Using this representation, a search unit having eight coils arranged andconnected as indicated at b in Fig. 5 Iwill give balance ofthe rst andsecond differentials in all directions of rotation. Similarly eightcoils arranged and connected as shown in Fig. 5 (c) will give balance ofall differentials in one direction but balance only of the firstdierential in another direction.

In Fig. 5 (a) it is assumed that (a) the sums of the areas enclosed bythe turns of the coils I3, I, I5 and I6 projected on to a planecontaining one of the turns, that is to say in this case a planeparallel to the plane of the paper, are equal. (b) the distances betweenthe centres of adjacent coils are equal, (c) the axis shown at 'Il isperpendicular to the axis joining the centres of the coils and isdisposed midway between the coils I4 and I5, and (d) measurements to theleft of the axis 'Il are negative and to the right of the axis 'il arepositive. The product of (a.) the sum of the areas enclosed by the turnsof one of the coils projected on to the plane containing one of theturns and (b) the distance of the centre of that coil from some axissuch as the axis ll, will be termed the area-turns moment of the coil.The sum of the areas enclosed by the turns of a coil, if all the turnsenclosed the same area, would of course be the product of the area andthe number of turns. On the basis of the above assumptions it can beshown that the algebraic sum of the area-turns moments of the coils I4and I5, forming one group, is equal to the algebraic sum of thearea-turns moments of the coils I3 and I6 forming the second group.

It will be appreciated that these algebraic sums are equal for any axisin a plane containing the centres of the coils. Furthermore it can beshown that these algebraic sums are also equal for the two groups ofcoils shown in each of Figs. 5 (b) and (c).

Although the coils shown in Fig. 5 are assumed to have equal numbers ofturns, to be equally spaced and to have the same area-turns, theseconditions are not essential for obtaining the desired results. Oneessential condition is that the sum of the areas enclosed by the turnsof the coils of one of the groups projected on to a plane containing oneof the turns must be substantially equal to the same sum for the coilsof the other group. In order to ensure that there is substantially zeroinput to the amplifier due to the rst space differential of the field,it is necessary to arrange that the algebraic sum of the area turnsmoments of the coils of the two groups are substantially equal. It willbe appreciated therefore, that coils having different dimensions andunequal numbers of turns may be used.

In the arrangements of Figs. 5 (b and (c), it can be arranged that theinput to the amplifier due to the second space differential of the eldis substantially zero by ensuring that the sum of the products of (a)the area-turns moments of the coils of one of the groups and (b) therespective distances used in determining the moments, is substantiallyequal to the same sum for the coils of the other group.

Other multiples of four coils than two may be employed, and by suitableconnection and disposition the required balance can'be obtained.

It is also possible to obtain the balance with two coils provided thatthey are arranged co-axially and'coplanar.

Thus, referring to Figs. 6 and 7, the search unit in this examplecomprises two coils 56 and 51 mounted co-axially and coplanar. The coil56 has three terminals 58, 59 and 60, and the coil 51 two terminals 6Iand 62, terminals 60 and 6l being connected together. The part of coil56 between terminals 58 and 59 serves as a. balancing coil the eiect ofwhich is adjustable by means of a, variable tap on a resistor 63. Thenumber of turns in, Iand the dimensions of, the coils are so arranged,according to well known formulae, that coil I51 and part of coil 56between terminal '60 and a mid-point between terminals 58 and 59 haveequal and opposite electro-motive forces induced in them when placed ina uniform magnetic field.

In order to reduce the input to the amplifier produced by rotation ofthe search unit about its axis normal to the plane of the coils, that isto say, perpendicular to the plane of the paper in Fig. 6, there may beprovided a number, for example four, coils 64, l65, 66 and 61 mounted inthe same plane as the coils 56 and 51 but arranged eccentrically withrespect to the axis of the coils 56 and 51. The eccentric coils 64 and65 are connected in series and deliver their voltages through areversing switch 68 and a potential divider 69. The eccentric coils 66and 61 are connected in series and deliver their voltages through areversing switch and potential divider 1l.

Coils I8 and I9 may also be provided in the manner and for the purposedescribed in connection with Fig. 2.

One form of search unit embodying coils as in Figs. 6 and 7 is shown inFig. 8 and is similar to that of Fig. 3. The search coils 56 and -51 arefixed to the housing part for example by adhesive. Eccentric coils, ofwhich'two are shown at 64 and 65, may be xed in like manner.

It is not essential in all cases to generate av magnetic eld in thesearch area. In some cases the body to be located, for instance a buriedcable carrying an alternating current, may itself generate a suilicientmagnetic field strength.

It will be appreciated that certain embodiments of the invention havebeen described by way of example only and may be modified in variousways within the scope of the invention as dened in the appended claims.Thus the principal search coils may be mounted horizontally as shown inFig. 3, vertically as shown in Fig. 9 or in some intermediate manner forexample as shown in lFig. 10. The coils, instead of being connected inseries-parallel, as in Fig. 2, may be connected in series as shown inFig. 6 or Fig. 10 or in parallel.

We claim:

1. Apparatus for the location of electrically conducting bodies andmagnetic bodies, comprising means including an electrically conductingloop and alternating current supply means connected to said loop, forproducing an alternating magnetic eld of substantial strength over alarge area, and search apparatus for searching over said area, saidsearch apparatus being movable relatively to said loop and including aplurality of search coils and a substantially nonmetallic structure forsupporting said coils in fixed relation to one another, said coil beingarranged in at least one set, each said set including a plurality ofsearch coils arranged in a row with their centers in a substantiallystraight line, an indicating device and means connecting said searchcoils in two groups to said indicating device, each coil of one of saidgroups being disposed between at least two of the coils of the other ofsaid groups, said connecting means serving to apply the voltages inducedin said two groups by said field in opposite senses to said indicatingdevice, the sum of the areas enclosed by all the individual coil turnsin one of said groups projected on to a reference plane containing oneof the turns of one of said coils, being equal to the same sum for theother of said groups, the same reference plane being used for all saidcoils, and the algebraic sum of the area-turns moments of the coils inone of said groups being equal to the algebraic sum of the area-turnsmoments of the coils in the other of said groups, said areaturns momentof` each of said coils being the product a, b, where a is the sum of theareas enclosed by all the individual turns of the coil projected on tosaid reference plane, and b is the distance of the center of the coilfrom any 'axis which cuts said linejoining the centers of said coils.

2. Apparatus for the location of electrically conducting bodies andmagnetic bodies, comprising means including an electrically conductingloop and alternating -current supply means connected to said loop, forproducing an alternating magnetic eld of substantial strength over alarge area, and search apparatus for searching over said area, saidsearch apparatus being movable relatively to said loop and including aplurality of search coils and a substantially non-metallic structure forsupporting said coils in xed relation to one another, said coils beingarranged in at least one set, each .said set including four search coilsarranged in a row with their centers substantially in a straight line,an indicating device, means connecting the two outer coils in said rowto said indicating device to form one group and means connecting the twoinner coils in said row to said indicating device to form a, secondgroup, said connecting means serving to apply the voltages induced insaid two groups in opposite senses to said indicating device, the sum ofthe areas enclosed by all the individual coil turns in one of saidgroups projected on to a reference plane containing one of the turns ofone of said coils, being equal to the same sum for the other of saidgroups, the same reference plane being used for all said coils, and thealgebraic sum of the area-turns moments of the coils in one of saidgroups being equal to the algebraic sum of the area-turns moment of thecoils in the other of said groups, said area-turns moment of each ofsaid coils being the produce a, b, where a is the sum of the areasenclosed by all the individual turns of the coil projected on to saidreference plane, and b is the distance of the centre of the coil fromany axis which cuts said line joining the centers of said coils.

3. Apparatus for the location of electrically conducting bodies andmagnetic bodies, comprising means including an electrically conductingloop and alternating vcurrent supply means connected to said loop, forproducing an alternating magnetic iield of substantial strength over alarge area, and search apparatus for searching over said area and havingdimensions which are small relatively to those of said loop, said searchapparatus being movable relatively to said loop and including foursubstantially equally-spaced search coils arranged in a substantiallystraight row, an indicating device, means connecting the two outer coilsin said row to said indicating device, and means connecting the twoinner coils in said row to said indicating device, said connecting meansserving to connect said two outer coils in opposition with said twoinner coils, said four search coils being of equal area turns, and theturns of all said coils lying in parallel planes.

4. Apparatus as claimed in claim 1, and comprising two compensatingcoils mounted upon said structure with their axes mutualy perpendicularand perpendicular to the 'axes of said search coils, and means forapplying a compensating voltage from said compensating coils to saidindicating device for the purpose of balancing.

5. Apparatus as claimed in claim l, wherein the indicating devicecomprises an amplifier, and wherein compensating means are provided,said compensating means being connected to said amplifier to render theoutput of said amplifier substantially independent of variations in themean strength of said altern-ating magnetic field.

6. Apparatus as claimed in claim 1, wherein said indicating devicecomprises an amplier,

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES lPATENTS Number Name Date 1,812,392 Zuschlag June 30, 19312,238,072 Nelson et al. Apr. 15, 1941 2,451,596 Wheeler Oct. 19, 1948

